Lithium iron phosphate of olivine crystal structure and lithium secondary battery using the same

ABSTRACT

Disclosed is lithium iron phosphate having an olivine crystal structure wherein carbon (C) is coated on particle surfaces of the lithium iron phosphate, wherein, when a powder of the lithium iron phosphate is dispersed in water, water is removed from the resulting dispersion and the resulting lithium iron phosphate residue is quantitatively analyzed, a ratio of the carbon-released lithium iron phosphate with respect to the total weight of the carbon-coated lithium iron phosphate is 0.005% by weight or less. Advantageously, the olivine-type lithium iron phosphate is not readily separated through uniform thin film coating on the surface of the lithium iron phosphate and exhibits superior conductivity and density, since carbon is coated on particle surfaces of lithium iron phosphate in a state in which the amount of carbon released in water is considerably small.

TECHNICAL FIELD

The present invention relates to lithium iron phosphate having anolivine crystal structure wherein carbon (C) is coated on particlesurfaces of the lithium iron phosphate, wherein, when a powder of thelithium iron phosphate is dispersed in water, water is removed from theresulting dispersion and the resulting lithium iron phosphate residue isquantitatively analyzed, a ratio of the carbon-released lithium ironphosphate with respect to the total weight of the carbon-coated lithiumiron phosphate is 0.005% by weight or less.

BACKGROUND ART

Technological development and increased demand for mobile equipment haveled to a rapid increase in the demand for secondary batteries as energysources. Among these secondary batteries, lithium secondary batterieshaving high energy density and voltage, long lifespan and lowself-discharge are commercially available and widely used.

The lithium secondary batteries generally use a carbon material as ananode active material. Also, the use of lithium metals, sulfurcompounds, silicon compounds, tin compounds and the like as the anodeactive material have been considered. Meanwhile, the lithium secondarybatteries generally use lithium cobalt composite oxide (LiCoO₂) as acathode active material. Also, the use of lithium-manganese compositeoxides such as LiMnO₂ having a layered crystal structure and LiMn₂O₄having a spinel crystal structure and lithium nickel composite oxide(LiNiO₂) as the cathode active material has been considered.

LiCoO₂ is currently used owing to superior physical properties such ascycle life, but has disadvantages of low stability and high-cost due touse of cobalt, which suffers from natural resource limitations, andlimitations of mass-use as a power source for electric automobiles.LiNiO₂ is unsuitable for practical application to mass-production at areasonable cost due to many features associated with preparation methodsthereof Lithium manganese oxides such as LiMnO₂ and LiMn₂O₄ have adisadvantage of short cycle life.

Accordingly, methods of using lithium transition metal phosphate as acathode active material have been researched. Lithium transition metalphosphate is largely divided into Li_(x)M₂(PO₄)₃ having a Nasiconstructure and LiMPO₄ having an olivine structure, and is found toexhibit superior high-temperature stability, as compared to conventionalLiCoO₂. To date, Li₃V₂(PO₄)₃ is the most widely known Nasicon structurecompound, and LiFePO4 and Li(Mn, Fe)PO₄ are the most widely studiedolivine structure compounds.

Among olivine structure compounds, LiFePO₄ has a high output voltage of3.5V and a high volume density of 3.6 g/cm³, as compared to lithium(Li), and has a high theoretical capacity of 170 mAh/g, exhibitssuperior high-temperature stability, as compared to cobalt (Co), andutilizes cheap Fe as an component, thus being highly applicable as thecathode active material for lithium secondary batteries.

However, LiFePO₄ disadvantageously causes an increase in internalresistance of batteries due to low electrical conductivity, when used asa cathode active material. For this reason, when battery circuits areclosed, polarization potential increases, thus decreasing batterycapacity.

In order to solve these problems, Japanese Patent ApplicationPublication No. 2001-110414 suggests incorporation of conductivematerials into olivine-type metal phosphates in order to improveconductivity.

However, LiFePO₄ is commonly prepared by solid state methods,hydrothermal methods and the like using Li₂CO₃ or LiOH as a lithiumsource. Lithium sources and carbon sources added to improve conductivitydisadvantageously generate a great amount of Li₂CO₃. Such Li₂CO₃ isdegraded during charging, or reacts with an electrolyte solution toproduce CO₂ gas, thus disadvantageously causing production of a greatamount of gas during storage or cycles. As a result, disadvantageously,swelling of batteries is generated and high-temperature stability isdeteriorated.

In this regard, a method for coating carbon on LiFePO₄ is known.However, through repeated experimentation, the inventors of the presentinvention found that a great amount of carbon should be used in order toobtain desired electrical conductivity using this method. Thedeterioration in overall physical properties is inevitable during designof batteries and further, a great amount of carbon used for coating ispresent in the form of aggregates between particles, thusdisadvantageously making it difficult to realize uniform coating.

Accordingly, there is an increasing need for methods to solve theseproblems.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the aboveproblems and other technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments, the inventors of the present invention have developedlithium iron phosphate that has an olivine crystal structure and iscoated with carbon in a state in which the amount of carbon released inwater is considerably small.

Technical Solution

In accordance with one aspect of the present invention, provided islithium iron phosphate having an olivine crystal structure whereincarbon (C) is coated on particle surfaces of the lithium iron phosphate,wherein, when a powder of the lithium iron phosphate is dispersed inwater, water is removed from the resulting dispersion and the resultinglithium iron phosphate residue is quantitatively analyzed, a ratio ofthe carbon-released lithium iron phosphate with respect to the totalweight of the carbon-coated lithium iron phosphate is 0.005% by weightor less.

A small amount of carbon is released from the olivine-type lithium ironphosphate according to the present invention, when dispersed in water,since carbon (C) is coated on the particle surface of the lithium ironphosphate through uniform and strong bonding force. Accordingly, thelithium iron phosphate can exhibit high electrical conductivity in spiteof coating of a small amount of carbon and prevent separation ofelectrodes in the manufacturing process thereof due to carbon coatingwith a strong bonding force, thus contributing to improvement inelectrode density.

In a preferred embodiment, the lithium iron phosphate may be lithiumiron phosphate having a composition represented by the following Formula(1).

Li_(1+a)Fe_(1−x)M_(x)(PO_(4−b))X_(b)   (1)

wherein

M is at least one selected from Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb,Zr, Ce, In, Zn and Y,

X is at least one selected from F, S and N, and

−0.5≦a≦+0.5, 0≦x≦0.5, 0≦b≦0.1.

Any type of compound may be used as an olivine-type lithium ironphosphate according to the present invention so long as it satisfies theconditions of the following formula 1 and a representative examplethereof is LiFePO₄, but is not limited thereto. It is impossible toobtain only pure LiFePO₄ in the preparation process of LiFePO₄.Characteristics required for the present invention can be exerted whenthe conditions of the following formula 1 are satisfied.

The bonding state of carbon to lithium iron phosphate can be confirmed,as defined above, by a method including dispersing a powder of lithiumiron phosphate in water for one hour, removing water from the resultingdispersion and measuring the weight of carbon-released lithium ironphosphate (lithium iron phosphate in which the carbon is released)through quantitative analysis of the resulting lithium iron phosphateresidue. More details thereof will be obtained from the following TestExample 1.

As defined above, the content of carbon-released lithium iron phosphateis 0.005 wt % or less with respect to the total weight of the carbon(C)-coated lithium iron phosphate, which is a considerably small amount,when taking into consideration the fact that, in the case ofconventional lithium iron phosphates, the ratio is 0.1 wt % or more whenanalyzed under the same conditions.

The content of the carbon-released lithium iron phosphate is preferably0.004 wt % or less.

Meanwhile, carbon (C) is preferably coated in an amount of 0.01 to 10%by weight, based on the weight of the lithium iron phosphate. When thecontent of carbon is excessive, the amount of active material becomesrelatively low, capacity disadvantageously decreases and electrodedensity is disadvantageously deteriorated. On the other hand, when thecontent of carbon is excessively small, disadvantageously, desiredelectrical conductivity cannot be obtained. The amount of coated carbonis more preferably 0.03 to 7% by weight.

In addition, carbon is preferably uniformly coated on the surface oflithium iron phosphate to a thickness of 2 to 50 nm. When carbon isexcessively thickly coated on the surface of lithium iron phosphate, itmay interfere with intercalation and deintercalation of lithium ions,and on the other hand, an excessively thin coating cannot secure uniformcoating and cannot provide desired electrical conductivity. A morepreferred coating thickness may be 3 to 10 nm.

In a preferred embodiment, the lithium iron phosphate may contain apredetermined amount of sulfur (S). The sulfur contained thereincontributes to coating of carbon on the lithium iron phosphate throughuniform and strong bonding force based on combined actions of sulfurwith carbon to lithium iron phosphate.

In the present invention, the relationship between sulfur and carbon isnot clear, but sulfur and carbon may be present in the form of astructure selected from (i) a structure in which carbon is coated on thesurface of lithium iron phosphate particles in a state in which apredetermined amount of sulfur is contained on the surface of lithiumiron phosphate particles and/or inside the same, (ii) a structure inwhich both sulfur and carbon are coated on the surface of lithium ironphosphate particles, (iii) a structure in which a composite of sulfurand carbon is coated on the surface of lithium iron phosphate particles,(iv) a structure in which carbon is bonded through sulfur to lithiumiron phosphate particles, and combinations thereof.

The sulfur (S) is preferably contained in an amount of 0.05 to 5% byweight, based on the total weight of the lithium iron phosphate.Disadvantageously, when the content of sulfur (S) is excessively high,physical properties of lithium iron phosphate may be deteriorated and,on the other hand, when the content of sulfur (S) is excessively low,uniform coating and strong bonding force of carbon cannot be exerted.The content is more preferably 0.1 to 2% by weight.

For example, the sulfur (S) may be derived from precursors forpreparation of lithium iron phosphate. When FePO₄ is used forpreparation of lithium iron phosphate, sulfur may remain in the productsafter reaction. Generally, when sulfur remains in an active material,washing is performed several times in order to completely remove thesulfur.

On the other hand, according to the present invention, remaining sulfur(S) may be used in combination with carbon coating, for example, after apredetermined amount of sulfur is left in lithium iron phosphate bydecreasing the repetition number of washing process or washingintensity, carbon coating is performed. For this reason, uniform andstrong coating as well as reduction of preparation costs of activematerial caused by simplification of washing process can beadvantageously realized. In particular, when primary particles oflithium iron phosphate are nanomaterials, the overall efficiency of thefabrication process of secondary batteries can be further improved byminimizing the washing process as described above.

In another embodiment, sulfur (S) may be incorporated by adding asulfur-containing compound to lithium iron phosphate. Thesulfur-containing compound is at least one selected from sulfides,sulfites and sulfates.

The present invention provides lithium iron phosphate having an olivinecrystal structure wherein the lithium iron phosphate has a compositionrepresented by the following Formula 2:

Li(1−a−b)Fe_(a/2)M′_(b/2)Fe_(1−c)M″_(c)P_(1−d)X_(d)O_(4−e)S_(e)   (2)

wherein M′ is at least one selected from the group consisting of Mg, Ni,Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr and Zn;

M″ is at least one selected from the group consisting of Al, Mg, Ni, Co,Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr and Zn;

X is at least one selected from the group consisting of As, Sb, Bi, Mo,V, Nb and Te; and

0≦a≦0.6, 0≦b≦0.6, 0≦c≦1, 0≦e≦3.5.

Considering the composition of Formula 2, Fe and/or M′ is doped intolithium sites and M″ is doped into Fe sites, X is doped into P sites X,and a part of sulfur (S) is substituted into oxygen sites.

In the formula above, a and b are each 0.6 or less, but when a+b is 1 orhigher, since lithium is not used, a+b should be lower than 1 duringcharge and discharge. a+b is preferably 0.5, in view of the amount oflithium that is intercalated and deintercalated during charge anddischarge and thus exhibits electrochemical properties.

The present invention provides a method for preparing the olivine-typelithium iron phosphate.

In a preferred embodiment, the olivine-type lithium iron phosphate maybe prepared by a method comprising:

(a) primarily mixing precursors as starting materials;

(b) secondarily mixing the mixture of step (a) with supercritical orsubcritical water to synthesize lithium iron phosphate;

(c) mixing the synthesized lithium iron phosphate with a carbonprecursor and drying the mixture; and

(d) heating the mixture of the lithium iron phosphate and the carbonprecursor.

In step (a), as a lithium precursor, one of the components, Li₂CO₃,Li(OH), Li(OH).H₂O, LiNO₃ or the like may be used. As an iron (Fe)precursor, a compound containing at least a sulfur ingredient so thatsulfur is left on the surface of the produced lithium iron phosphatesuch as FeSO₄, FeC₂O₄.2H₂O or FeCl₂ may be used. FeSO₄ is particularlypreferred since it contains a sulfur element. As a phosphorus (P)precursor, H₃PO₄, NH₄H₂PO₄, (NH₄)₂HPO₄ or P₂O₅ may be used.

If necessary, an alkalinizing agent may be further added to thecomponents. In this case, the allcalinizing agent may be an alkali metalhydroxide, an alkaline earth metal hydroxide, an ammonia compound or thelike.

In step (b), the supercritical or subcritical water may be water at apressure of 180 to 550 bar at 200 to 700° C. and the heating temperaturein step (d) may be 600 to 1200° C.

Any carbon precursor may be used so long as it can produce carbon duringa baking process under a reduction atmosphere. Preferably, the carbonprecursor may be a polyol-type carbon-containing precursor andnon-limiting examples thereof include sucrose, cellulose, glucose andthe like.

In another embodiment, the olivine-type lithium iron phosphate may beprepared by the following method comprising:

(a′) primarily mixing precursors as starting materials;

(b′) secondarily mixing the mixture of step (a′) with supercritical orsubcritical water to synthesize lithium iron phosphate, followed bydrying;

(c′) heating the synthesized lithium iron phosphate; and

(d′) milling the lithium iron phosphate and a carbon powder.

In step (d′), the milling method is well-known in the art and a detailedexplanation thereof is thus omitted. In a preferred embodiment, themilling method may be ball milling.

In step (d) or (c′), the heating may be carried out under an inert gasatmosphere. Any inert gas may be used without particular limitation solong as it has low reactivity. Preferred examples thereof include Ar, N₂and the like.

The synthesis of lithium iron phosphate of the present invention ispreferably performed through a continuous reaction process.

The lithium iron phosphate according to the present invention may be inthe form of a primary particle or a secondary particle. The lithium ironphosphate in the form of a secondary particle may be prepared by dryinga mixture of primary particles having a predetermined particle diameter,a binder and a solvent, followed by aggregating.

In the mixture, preferably, the primary particle is present in an amountof 5 to 20 wt % with respect to the weight of the solvent and the binderis present in an amount of 5 to 20 wt% with respect to the weight of thesolvent. By controlling a ratio of the primary particle and the solvent,an internal porosity of the secondary particle can be controlled.Examples of the solvent that can be used during the process include allorganic solvents such as polar solvents such as water and non-polarsolvents. In addition, examples of the binder used in the step include,but are not limited to, sucrose and lactose-based sugars, PVDF orPE-based polymers and cokes which are soluble in a polar solvent.

The dying and preparation of the secondary particles may be carried outat the same time by various methods known in the art, including spraydrying, fluidized-bed drying, vibration drying, etc. In particular,rotating spray drying is preferred, because it enables preparation ofsecondary particles in the form of spheres and thus improves tapdensity.

The drying may be carried out at 120 to 200° C. under inert gas (e.g.,Ar, N₂) atmosphere.

Also, the olivine-type lithium iron phosphate according to the presentinvention is preferably prepared by a coprecipitation or solid phasemethod.

For example, the olivine-type lithium iron phosphate containing sulfur(S) may be prepared by a method comprising:

(a″) synthesizing lithium iron phosphate using precursors as startingmaterials by a coprecipitation or solid phase method;

(b″) adding the synthesized lithium iron phosphate to a dispersionchamber containing a sulfur-containing compound, followed by stirring;

(c″) drying the mixture obtained in step (b″), followed by baking; and

(d″) mixing the lithium iron phosphate obtained in step (c″) with acarbon powder, followed by milling, or mixing the calcinized lithiumiron phosphate and a carbon precursor with a solvent, followed by dryingand baking.

The coprecipitation or solid phase method in step (a″) is well-known inthe art and a detailed explanation thereof is thus omitted.

The sulfur-containing compound used for step (b″) may be a sulfide,sulfite, sulfate or the like, as mentioned above, and may be used withina range in which a content suggested in the present invention of sulfurcan be contained.

The present invention provides a cathode mix comprising lithium ironphosphate as a cathode active material. The cathode mix may optionallycontain, in addition to the cathode active material, a conductivematerial, a binder, a filler and the like.

The conductive material is commonly added in an amount of 1 to 30% byweight, based on the total weight of the mixture comprising the cathodeactive material. Any conductive material may be used without particularlimitation so long as it has suitable conductivity without causingadverse chemical changes in the battery. Examples of conductivematerials include conductive materials, including graphite; carbonblacks such as carbon black, acetylene black, Ketjen black, channelblack, furnace black, lamp black and thermal black; conductive fiberssuch as carbon fibers and metallic fibers; metallic powders such ascarbon fluoride powders, aluminum powders and nickel powders; conductivewhiskers such as zinc oxide and potassium titanate; conductive metaloxides such as titanium oxide; and polyphenylene derivatives.

The binder is a component which enhances binding of an electrode activematerial to a conductive material and current collector. The binder iscommonly added in an amount of 1 to 30% by weight, based on the totalweight of the mixture comprising the cathode active material. Examplesof the binder include polyvinylidene, polyvinyl alcohol,carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene propylene diene terpolymer (EPDM),sulfonated EPDM, styrene butadiene rubbers, fluororubbers and variouscopolymers.

The filler is a component optionally used to inhibit expansion of theelectrode. Any filler may be used without particular limitation so longas it does not cause adverse chemical changes in the manufacturedbattery and is a fibrous material. Examples of the filler include olefinpolymers such as polyethylene and polypropylene; and fibrous materialssuch as glass fibers and carbon fibers.

Meanwhile, the cathode active material may be composed of only theolivine-type lithium iron phosphate and a combination of olivine-typelithium iron phosphate and lithium-containing transition metal oxide, ifnecessary.

Examples of the lithium transition metal composite oxide include, butare not limited to, layered compounds such as lithium cobalt oxide(LiCoO₂) and lithium nickel oxide (LiNiO₂), or compounds substitutedwith one or more transition metals; lithium manganese oxides such ascompounds of formula Li_(1+y)Mn_(2−y)O₄ (0≦y≦0.33), LiMnO₃, LiMn₂O₃ andLiMnO₂; lithium copper oxide (Li₂CuO₂); vanadium oxides such as LiV₃O₈,LiFe₃O₄, V₂O₅ and Cu₂V₂O₇; Ni-site type lithiated nickel oxides ofFormula LiNi_(1−y)M_(y)O₂ (M=Co, Mn, Al, Cu, Fe, Mg, B or Ga, and0.01≦y≦0.3); lithium manganese composite oxides of FormulaLiMn_(2−y)M_(y)O₂ (M=Co, Ni, Fe, Cr, Zn or Ta, and 0.01≦y≦0.1), orformula Li₂Mn₃MO₈ (M=Fe, Co, Ni, Cu or Zn); LiMn₂O₄ wherein a portion ofLi is substituted with alkaline earth metal ions; disulfide compounds;and Fe₂(MoO₄)₃.

The present invention provides a cathode wherein the cathode mix isapplied to a collector.

The cathode for secondary batteries may be prepared by applying a slurryobtained by mixing the cathode mix with a solvent such as NMP to acathode current collector, followed by drying and press-rolling.

The cathode current collector is generally fabricated to have athickness of 3 to 500 μm. There is no particular limit as to the cathodecurrent collector, so long as it has suitable conductivity withoutcausing adverse chemical changes in the fabricated battery. Examples ofthe cathode current collector include stainless steel, aluminum, nickel,titanium, sintered carbon, and aluminum or stainless steel which hasbeen surface-treated with carbon, nickel, titanium or silver. Ifnecessary, these current collectors may also be processed to form fineirregularities on the surface thereof so as to enhance adhesive strengthto the cathode active materials. In addition, the current collectors maybe used in various forms including films, sheets, foils, nets, porousstructures, foams and non-woven fabrics.

The present invention provides a lithium secondary battery comprisingthe cathode, the anode, a separator, and a lithium salt-containingnon-aqueous electrolyte.

For example, the anode is prepared by applying an anode mix comprisingan anode active material to an anode current collector, followed bydrying. The anode mix may comprise the afore-mentioned components, i.e.,the conductive material, the binder and the filler, if necessary.

The anode current collector is generally fabricated to have a thicknessof 3 to 500 μm. There is no particular limit as to the anode currentcollector, so long as it has suitable conductivity without causingadverse chemical changes in the fabricated battery. Examples of theanode current collector include copper, stainless steel, aluminum,nickel, titanium, sintered carbon, and copper or stainless steel whichhas been surface-treated with carbon, nickel, titanium or silver, andaluminum-cadmium alloys. Similar to the cathode current collector, ifnecessary, these current collectors may also be processed to form fineirregularities on the surface thereof so as to enhance adhesive strengthto the anode active materials. In addition, the current collectors maybe used in various forms including films, sheets, foils, nets, porousstructures, foams and non-woven fabrics.

Examples of the anode active material include carbon and graphitematerials such as natural graphite, artificial graphite, expandedgraphite, carbon fiber, hard carbon, carbon black, carbon nanotubes,perylene, activated carbon; metals alloyable with lithium, such as Al,Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti and compoundscontaining these elements; composites of carbon and graphite materialswith a metal and a compound thereof; and lithium-containing nitrides. Ofthese, a carbon-based active material, a silicon-based active material,a tin-based active material, or a silicon-carbon-based active materialis more preferred. The material may be used alone or in combination oftwo or more thereof.

The separator is interposed between the cathode and anode. As theseparator, an insulating thin film having high ion permeability andmechanical strength is used. The separator typically has a pore diameterof 0.01 to 10 μand a thickness of 5 to 300 μm. As the separator, sheetsor non-woven fabrics made of an olefin polymer such as polypropyleneand/or glass fibers or polyethylene, which have chemical resistance andhydrophobicity, are used. When a solid electrolyte such as a polymer isemployed as the electrolyte, the solid electrolyte may also serve asboth the separator and electrolyte.

The lithium salt-containing, non-aqueous electrolyte is composed of anon-aqueous electrolyte and a lithium salt. As the non-aqueouselectrolyte, a non-aqueous electrolyte solution, solid electrolyte andinorganic solid electrolyte may be utilized.

Examples of the non-aqueous electrolyte solution that can be used in thepresent invention include non-protic organic solvents such asN-methyl-2-pyrollidinone, propylene carbonate, ethylene carbonate,butylene carbonate, dimethyl carbonate, diethyl carbonate,gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydroxy Franc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trim ethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate and ethylpropionate.

Examples of the organic solid electrolyte utilized in the presentinvention, include polyethylene derivatives, polyethylene oxidederivatives, polypropylene oxide derivatives, phosphoric acid esterpolymers, poly agitation lysine, polyester sulfide, polyvinyl alcohols,polyvinylidene fluoride, and polymers containing ionic dissociationgroups.

Examples of the inorganic solid electrolyte include nitrides, halidesand sulfates of lithium such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH,LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH andLi₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in theabove-mentioned non-aqueous electrolyte and examples thereof includeLiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroboranelithium, lower aliphatic carboxylic acid lithium, lithium tetraphenylborate and imides.

Additionally, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride or the like may be added to the non-aqueouselectrolyte. If necessary, in order to impart incombustibility, thenon-aqueous electrolyte may further contain halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristics, the non-aqueouselectrolyte may further contain carbon dioxide gas or the like and mayfurther contain fluoro-ethylene carbonate (FEC), propene sultone (PRS),fluoro-ethylene carbonate (FEC) and the like.

The lithium secondary batteries according to the present invention maybe used as unit batteries of battery modules, which are power sources ofmedium and large devices requiring high-temperature stability, longcycle properties and high rate properties.

Preferably, examples of medium and large devices include power toolspowered by battery-driven motors; electric vehicles including electricvehicles (EVs), hybrid electric vehicles (HEVs) and plug-in hybridelectric vehicles (PHEVs); electric two-wheeled vehicles includingelectric bikes (E-bikes), electric scooters (E-scooter); electric golfcarts and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 shows XRD analysis data of a grayish red-brown residue obtainedafter lithium iron phosphate of Comparative Example 1 is stirred in asolvent in Test Example 2.

BEST MODE

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided only toillustrate the present invention and should not be construed as limitingthe scope and spirit of the present invention.

EXAMPLE 1

21.5 g of LiOH—H₂O, 22 g of aqueous ammonia (˜29 wt %), and 956.5 g ofdistilled water were mixed with one another and dissolved to prepare anaqueous solution A. In the same manner as above, 70.65 g of FeSO₄—7H₂O,7.065 g of sucrose, 28.85 g of phosphoric acid (85 wt %), and 893.435 gof distilled water were mixed with one another and dissolved to preparean aqueous solution B.

Supercritical water (450° C., 250 bar) was flowed at an elevatedtemperature and at an elevated pressure at 100 g/min into a continuoustubular reactor, and the aqueous solution A and the aqueous solution Bwere flowed at a flow rate of 15 g/min and brought in contact with thesupercritical water for several seconds and mixed to induce reaction. Atthis time, the aqueous solution A contacted the aqueous solution B toproduce a slurry and was then reacted with the supercritical water. Theaqueous solution A was reacted with supercritical water within onesecond after production of slurry.

The resulting product was cooled and filtered at the end of the tubularreactor to obtain a slurry containing an intermediate. A controlledconcentration of water was added to the slurry to obtain a slurry havinga solid content of 20 wt %, and 10 wt % of sucrose based on the solidwas added thereto, followed by dissolution. The slurry thus obtained wasspray-dried to obtain a sucrose-coated LiFePO₄ powder. The powder thusobtained was heated under a nitrogen atmosphere at about 700° C. for 10hours to obtain a final carbon-coated LiFePO₄ powder. As a result ofXRD-Rietveld analysis, it could be seen that the powder was a LiFePO₄crystal. The LiFePO₄ powder thus obtained was subjected to C&S analysisto measure contents of carbon and sulfur. As a result, the content ofcarbon was found to be 2.5 wt % and the content of sulfur was found tobe 0.74 wt %.

EXAMPLE 2

21.5 g of LiOH—H₂O, 27 g of aqueous ammonia (˜29 wt %), and 950.5 g ofdistilled water were mixed with one another and dissolved to prepare anaqueous solution A. In the same manner as above, 72.65 g of FeSO₄—7H₂O,6.065 g of sucrose, 28.85 g of phosphoric acid (85 wt%), and 890.435 gof distilled water were mixed with one another and dissolved to preparean aqueous solution B.

Supercritical water (430° C., 240 bar) was flowed at an elevatedtemperature and at an elevated pressure at 100 g/min into a continuoustubular reactor, and the aqueous solution A and the aqueous solution Bwere flowed at a flow rate of 15 g/min and brought in contact with thesupercritical water for several seconds and mixed to induce reaction.

The resulting product was cooled and filtered at the end of the tubularreactor to obtain a slurry containing an intermediate. A controlledconcentration of water was added to the slurry to obtain a slurry havinga solid content of 20 wt %, and 7 wt % of sucrose based on the solid wasadded thereto, followed by dissolution. The slurry thus obtained wasspray-dried to obtain a sucrose-coated LiFePO₄ powder. The powder thusobtained was heated under a nitrogen atmosphere at about 700° C. for 10hours to obtain a final carbon-coated LiFePO₄ powder. As a result ofXRD-Rietveld analysis, it could be seen that the powder was a LiFePO₄crystal. The LiFePO₄ powder thus obtained was subjected to C&S analysisto measure contents of carbon and sulfur. As a result, the content ofcarbon was found to be 1.8 wt % and the content of sulfur was found tobe 0.82 wt %.

EXAMPLE 3

40.9 g of LiOH—H₂O, 32.4 g of aqueous ammonia (˜29 wt%), and 934.7 g ofdistilled water were mixed with one another and dissolved to prepare anaqueous solution A. In the same manner as above, 140.3 g of FeSO₄—7H₂O,13.13 g of sucrose, 57.7 g of phosphoric acid (85 wt %), 3.1 g of sulfurand 767 g of distilled water were mixed with one another and dissolvedto prepare an aqueous solution B. Supercritical water (450° C., 250 bar)was flowed at an elevated temperature and at an elevated pressure at 100g/min into a continuous tubular reactor, and the aqueous solution A andthe aqueous solution B were flowed at a flow rate of 15 g/m in andbrought in contact with the supercritical water for several seconds andmixed to induce reaction. At this time, the aqueous solution A firstcontacted the aqueous solution B to produce a slurry and was thenreacted with the supercritical water. The aqueous solution A was reactedwith supercritical water as soon as possible after production of slurryto prepare LiFePO_(3.8)S_(0.2).

The LiFePO_(3.8)S_(0.2) reaction solution thus obtained was cooled andfiltered at the end of the tubular reactor to obtain aLiFePO_(3.8)S_(0.2) slurry. A controlled concentration of water wasadded to the slurry to obtain a slurry having a solid content of 10 wt%, and 10 w t% of sucrose based on the solid was added thereto, followedby dissolution. The slurry thus obtained was spray-dried to obtain asucrose-coated LiFePO_(3.8)S_(0.2) powder.

The powder thus obtained was heated under a nitrogen atmosphere at about700° C. for 10 hours to obtain a final carbon-coated LiFePO_(3.8)S_(0.2)powder.

The LiFePO_(3.8)S_(0.2) powder thus obtained was subjected to C&Sanalysis to measure contents of carbon and sulfur. As a result, thecontent of carbon was found to be 1.8 wt % and the content of sulfur wasfound to be 0.9 wt %.

EXAMPLE 4

34.8 g of LiOH—H₂O, 37.1 g of aqueous ammonia (˜29 wt %), and 904.7 g ofdistilled water were mixed with one another and dissolved to prepare anaqueous solution A. In the same manner as above, 104.2 g of FeSO₄—7H₂O,14.13 g of sucrose, 57.7 g of phosphoric acid (85 wt %), 15.3 g ofNiSO₄—7H₂O and 795.7 g of distilled water were mixed with one anotherand dissolved to prepare an aqueous solution B. Supercritical water(455° C., 240 bar) was flowed at an elevated temperature and at anelevated pressure at 100 g/min into a continuous tubular reactor, andthe aqueous solution A and the aqueous solution B were flowed at a flowrate of 15 g/min and brought in contact with the supercritical water forseveral seconds and mixed to induce reaction. At this time, the aqueoussolution A first contacted the aqueous solution B to produce a slurryand was then reacted with the supercritical water. The aqueous solutionA was reacted with supercritical water as soon as possible afterproduction of slurry to prepare Li_(0.7)Ni_(0.15)Fe_(0.78)Ni_(0.22)PO₄.

The Li_(0.7)Ni_(0.15)Fe_(0.78)Ni_(0.22)PO₄ reaction solution thusobtained was cooled and filtered at the end of the tubular reactor toobtain Li_(0.7)Ni_(0.15)Fe_(0.78)Ni_(0.22)PO₄ slurry. A controlledconcentration of water was added to the slurry to obtain a slurry havinga solid content of 10 wt %, and 12 wt % of sucrose based on the solidwas added thereto, followed by dissolution. The slurry thus obtained wasspray-dried to obtain a sucrose-coatedLi_(0.7)Ni_(0.15)Fe_(0.78)Ni_(0.22)PO₄ powder.

The powder thus obtained was heated under a nitrogen atmosphere at about650° C. for 10 hours to obtain a final carbon-coatedLi_(0.7)Ni_(0.15)Fe_(0.78)Ni_(0.22)PO₄ powder.

The Li_(0.7)Ni_(0.15)Fe_(0.78)Ni_(0.22)PO₄ powder thus obtained wassubjected to C&S analysis to measure contents of carbon and sulfur. As aresult, the content of carbon was found to be 2.7 wt % and the contentof sulfur was found to be 0.7 wt %.

EXAMPLE 5

34.1 g of LiOH—H₂O, 30.4 g of aqueous ammonia (˜29 wt %), and 914.7 g ofdistilled water were mixed with one another and dissolved to prepare anaqueous solution A. In the same manner as above, 103.3 g of FeSO₄—7H₂O,17.13 g of sucrose, 58.7 g of phosphoric acid (85 wt %), 9.2 g ofNiSO₄—7H₂O, 11 g of CaCl₂—6H₂O and 756.2 g of distilled water were mixedwith one another and dissolved to prepare an aqueous solution B.Supercritical water (410° C., 230 bar) was flowed at an elevatedtemperature and at an elevated pressure at 100 g/min into a continuoustubular reactor, and the aqueous solution A and the aqueous solution Bwere flowed at a flow rate of 15 g/min and brought in contact with thesupercritical water for several seconds and mixed to induce reaction. Atthis time, the aqueous solution A first contacted the aqueous solution Bto produce a slurry and was then reacted with the supercritical water.The aqueous solution A was reacted with supercritical water as soon aspossible after production of slurry to prepareLi_(0.7)Ca_(0.15)Fe_(0.79)Ni_(0.15)Ca_(0.06)PO₄.

The Li_(0.3)Ca_(0.15)Fe_(0.79)Ni_(0.15)Ca_(0.06)PO₄ reaction solutionthus obtained was cooled and filtered at the end of the tubular reactorto obtain a Li_(0.3)Ca_(0.15)Fe_(0.79)Ni_(0.15)Ca_(0.06)PO₄ slurry. Acontrolled concentration of water was added to the slurry to obtain aslurry having a solid content of 15 wt %, and 6 wt % of sucrose based onthe solid was added thereto, followed by dissolution. The slurry thusobtained was spray-dried to obtain a sucrose-coatedLi_(0.7)Ca_(0.15)Fe_(0.79)Ni_(0.15)Ca_(0.06)PO₄ powder.

The powder thus obtained was heated under a nitrogen atmosphere at about720° C. for 7 hours to obtain a final carbon-coatedLi_(0.3)Ca_(0.15)Fe_(0.79)Ni_(0.15)Ca_(0.06)PO₄ powder.

The Li_(0.3)Ca_(0.15)Fe_(0.79)Ni_(0.15)Ca_(0.06)PO₄ powder thus obtainedwas subjected to C&S analysis to measure contents of carbon and sulfur.As a result, the content of carbon was found to be 1.3 wt % and thecontent of sulfur was found to be 0.5 wt %.

COMPARATIVE EXAMPLE 1

LiOH—H₂O, Fe(C₂O₄)—2H₂O, and H₃PO₄ were placed as raw materials at amolar ratio of 3:1:1 in an autoclave batch reactor, and the materialswere reacted with one another at an elevated internal temperature of thereactor of about 200° C. for 6 hours to synthesize LiFePO₄. Sucrose wasadded to the slurry containing LiFePO₄ thus obtained, in the same manneras Example 1, followed by spray-drying and baking under a nitrogenatmosphere to obtain a LiFePO₄ powder as a final product. The LiFePO₄powder thus obtained was subjected to C&S analysis to measure contentsof carbon and sulfur. As a result, the content of carbon was found to be2 wt % and the content of sulfur was found to be 0.01 wt %.

TEST EXAMPLE 1

100 g of each of the lithium iron phosphate powders prepared in Examples1 to 5 and Comparative Example 1 was added to 500 g of water, followedby stirring for one hour and dispersing. Then, the dispersion state wasconfirmed. As a result, the dispersions of lithium iron phosphatepowders according to Examples 1 to 5 were transparent in color, whilethe dispersion of the lithium iron phosphate powder according toComparative Example 1 was gray in color.

The dispersion was separated, water was removed therefrom bycentrifugation, dried at 80° C. for 12 hours to obtain grayish red-brownresidues. It was confirmed through XRD analysis that the residues werepowders having a pure olivine crystal structure. The XRD analysis datais shown in FIG. 1.

In addition, presence of black particles on the surface of thedispersion was observed. It was confirmed through analysis that theparticles were carbon particles. As apparent from the fore-going, whenlithium iron phosphate powders prepared in Examples 1 to 5 andComparative Example 1 were dispersed in water, carbon was separated(released) from the carbon-coated lithium iron phosphate.

After the lithium iron phosphate powder of Comparative Example 1 wassubjected to dispersion testing for one hour, the grayish red-brownresidue was weighed. As a result, the weight was 0.12 g, which meansthat a ratio of the weight of the carbon-released lithium iron phosphateparticles with respect to the total weight of the carbon-coated lithiumiron phosphate particles is 0.12 wt %. The lithium iron phosphatepowders of Examples 1 to 5 were subjected to dispersion testing and theresulting residues were weighed. As a result, the weights were 0.003 g,0.001 g, 0.002 g, 0.003 g and 0.002 g, which means that respectiveratios of the weight of the carbon-released lithium iron phosphateparticles with respect to the total weight of the carbon-coated lithiumiron phosphate were 0.003 wt %, 0.001 wt %, 0.002 wt %, 0.003 wt % and0.002 wt %.

As apparent from the fore-going, the powder of Comparative Example 1exhibited considerably superior carbon-release property in water, ascompared to the powders of Examples 1 to 5.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

INDUSTRIAL APPLICABILITY

As apparent from the afore-going, advantageously, the olivine-typelithium iron phosphate according to the present invention is not readilyseparated through uniform thin film coating on the surface of thelithium iron phosphate and exhibits superior conductivity and density,since carbon is coated on particle surfaces of lithium iron phosphate ina state in which the amount of carbon released in water is considerablysmall.

1. Lithium iron phosphate having an olivine crystal structure whereincarbon (C) is coated on particle surfaces of the lithium iron phosphate,wherein, when a powder of the lithium iron phosphate is dispersed inwater, water is removed from the resulting dispersion and the resultinglithium iron phosphate residue is quantitatively analyzed, a ratio ofthe carbon-released lithium iron phosphate with respect to the totalweight of the carbon-coated lithium iron phosphate is 0.005% by weightor less.
 2. The lithium iron phosphate according to claim 1, wherein thelithium iron phosphate has a composition represented by the followingFormula 1:Li_(1+a)Fe_(1−x)M_(x)(PO_(4−b))X_(b)   (1) wherein M is at least oneselected from Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn andY, X is at least one selected from F, S and N, and−0.5≦a≦+0.5, 0≦x≦0.5, 0≦b≦0.1.
 3. The lithium iron phosphate accordingto claim 2, wherein the lithium iron phosphate is LiFePO₄.
 4. Thelithium iron phosphate according to claim 1, wherein a ratio of thecarbon-released lithium iron phosphate with respect to the total weightof the carbon-coated lithium iron phosphate is 0.004% by weight or less.5. The lithium iron phosphate according to claim 1, wherein the carbon(C) is coated at an amount of 0.01 to 10% by weight, based on the totalweight of the lithium iron phosphate.
 6. The lithium iron phosphateaccording to claim 1, wherein the carbon is coated on the surface oflithium iron phosphate to a thickness of 2 to 50 nm.
 7. The lithium ironphosphate according to claim 1, wherein the lithium iron phosphatecontains 0.05 to 5% by weight of sulfur (S), based on the total weightof the lithium iron phosphate.
 8. The lithium iron phosphate accordingto claim 7, wherein the sulfur and the carbon are present in the form ofa structure selected from the group consisting of (i) a structure inwhich carbon is coated on the surface of lithium iron phosphateparticles in a state in which a predetermined amount of sulfur iscontained on the surface of lithium iron phosphate particles and/orinside the same, (ii) a structure in which both sulfur and carbon arecoated on the surface of lithium iron phosphate particles, (iii) astructure in which a composite of sulfur and carbon is coated on thesurface of lithium iron phosphate particles, (iv) a structure in whichcarbon is bonded through sulfur to lithium iron phosphate particles, andcombinations thereof.
 9. The lithium iron phosphate according to claim1, wherein the lithium iron phosphate has a composition represented bythe following Formula 2:Li_((1−a−b))Fe_(a/2)M′_(b/2)Fe_(1−c)M″_(c)P_(1−d)X_(d)O_(4−e)S_(e)   (2)wherein M′ is at least one selected from the group consisting of Mg, Ni,Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr and Zn; M″ is at least oneselected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Cr, Cu, V,Ce, Sn, Ba, Ca, Sr and Zn; X is at least one selected from the groupconsisting of As, Sb, Bi, Mo, V, Nb and Te; and0≦a≦0.6, 0≦b≦0.6, 0≦c≦1, 0≦e≦3.5.
 10. A method for preparing the lithiumiron phosphate according to claim 1 comprising: (a) primarily mixingprecursors as starting materials; (b) secondarily mixing the mixtureobtained in step (a) with supercritical or subcritical water tosynthesize lithium iron phosphate; (c) mixing the synthesized lithiumiron phosphate with a carbon precursor and drying the mixture; and (d)heating the mixture of the lithium iron phosphate and the carbonprecursor.
 11. A method for preparing the lithium iron phosphateaccording to claim 1 comprising: (a′) primarily mixing precursors asstarting materials; (b) secondarily mixing the mixture obtained in step(a′) with supercritical or subcritical water to synthesize lithium ironphosphate, followed by drying; (c′) heating the synthesized lithium ironphosphate; and (d′) milling the lithium iron phosphate and a carbonpowder.
 12. The method according to claim 10, wherein the heating iscarried out under an inert gas atmosphere.
 13. The method according toclaim 10, wherein the synthesis of the lithium iron phosphate is carriedout by a continuous reaction process.
 14. A cathode mix comprising thelithium iron phosphate according to claim 1 as a cathode activematerial.
 15. A lithium secondary battery comprising a cathode in whichthe cathode mix according to claim 14 is applied to a current collector.16. The lithium secondary battery according to claim 15, wherein thelithium secondary battery is a unit battery of a battery module that isa power source of a medium to large device.
 17. The lithium secondarybattery according to claim 16, wherein the medium to large device is apower tool, an electric vehicle, a hybrid electric vehicle, or anelectric golf cart.